CARCINOMA OF PROSTATE II: METASTASES
In 10% of patients at presentation, prostatic carcinoma reveals contiguous spread to other organs (see Plate 4-10). However, with aggressive early detection strategies currently used, far fewer patients exhibit metastases to distant sites. When it occurs, prostate cancer has an overwhelming predilection to metastasize to bone. It is thought that this unique metastatic pattern is due to the fact that (1) circulating prostatic cancer cells may be found early on in the life span of the cancer and (2) metastatic cells tend to be arrested in cortical and medullary bone spaces. Tumor expansion in bone may cause pain, compression, pathologic fractures, and anemia due to bone marrow replacement.
The pattern of bony metastases in prostate cancer also occurs in a fairly
characteristic manner, with involvement of both the axial and appendicular
skeletons, typically the pelvis, sacrum, and spine, observed most commonly.
This metastatic distribution may be a consequence of pelvic venous drainage
through Batson plexus, a network of valveless veins that connect the deep
pelvic veins draining the inferior bladder, prostate, and rectum to the
internal vertebral venous plexuses. Because of their location and valveless
nature, they are thought to provide a route for the spread of cancer metastases
from prostate and colorectal cancers. From this landing zone, cancer may then
spread to the vertebral column or brain. The plexus is named after anatomist
Oscar Vivian Batson, who first described it in 1940. The sites of bone involvement
are illustrated in the frequency with which they occur.
Prostate cancer involvement of visceral and soft tissue nodal sites is
less common than bony metastases. In patients with hormone-refractory prostate
cancer, bony involvement can be found in 85% of patients, soft tissue or nodal
involvement in 25%, and visceral metastases (mainly to lung and liver) in 18%.
Although not as characteristic as bony metastases, the approximate sites of
visceral and soft tissue nodal involvement are also illustrated in the frequency
with which they occur. Bony metastases from prostate cancer, when viewed on
plain x-rays, appear osteoblastic. The metastasis has a “snowy” appearance
because of an increased deposition of calcium. A more destructive osteoclastic
process (osteolytic metastases) occurs in 2% of cases. The pathologic fracture
rate from prostate cancer is relatively low compared to that of other
metastatic cancers precisely because it induces bone-forming osteoblastic
reactions. Even when fractures occur, they heal similarly to those of normal
bone, limiting the need for surgical stabilization. Elevation of the serum acid
phosphatase level is found in two-thirds of patients with metastases and is
usually elevated when osteoblastic metastases are
present.
The value of a nuclear bone scan to detect bony metastases is limited to
patients with a Gleason sum score lower than 7 and a prostate-specific antigen
level lower than 20 ng/mL. A bone scan is also indicated in patients with
prostate cancer who have symptoms suggesting bony metastases, but bone scan
activity may not be observed until 5 years
after micrometastases have developed. Abdominal or pelvic computed tomographic
(CT) or magnetic resonance imaging (MRI) scans may reveal extracapsular
extension, seminal vesical involvement, pelvic lymph node enlargement, liver
metastases, and hydronephrosis (due to ureteral obstruction) in patients
suspected of having locally advanced disease. Immunoscintigraphy can also be
used to reveal extraprostatic disease, but these scans frequently yield false-negative results; specificity is improved when combined with CT
scans. Depending on the urgency and need for treatment, bony metastases from
prostate cancer are treated with androgen deprivation therapy in naïve tumors
and generally respond to radiation therapy. Exciting clinical trials with novel
chemotherapy agents and prostate cancer vaccines are under way as the survival
rate for hormone-refractory, metastatic prostate cancer is grim and projected
at 8% to 10% at 5 years.
